Copper oxidation



Dec. 17, 1963 H. CHORNEY 3,114,683

COPPER OXIDATION Filed Oct. 7, 1960 POTENTIAL OF SHEET +2.0 VOLTS,

DC FIG. 2 0 j 30 60 9 TIME, U O SECONDS SUBMERGED LENGTH PASSlVE BLACKENING PORTION PORTlON FIG. 3

INVENTOR. HARRY CHORNEY BY @3147 6 ATTORNEY ted Snares atnt h Bilifidd Patented Dec. 17, 1963 3,114,683 CGPPER OXIDATION Harry Chorney, Brookline, NH, assignor to Sanders Associates, Inc, Nashua, N.H., a corporation of Delaware Filed on. 7, 1960, Set. N .smss 1 Claim. at. 204-28) This invention relates to the oxidation of copper to form a surface layer of black copper oxide on sheets or similar wide continuous lengths of copper, and provides a process of forming copper oxide coatings on sheets by anodic oxidation in a continuous process.

In forming articles of copper, particularly electrical circuit elements, it is frequently the practice to oxidize the surface of the copper to form a black oxide coating which serves to improve the adhesion of the copper to materials, particularly plastics, which are applied to the copper. Also, since metallic copper frequently exerts a deleterious effect on plastics, an oxide coating serves to protect the plastic from such action. Copper sheets used to form base panels for printed electronic circuits may be provided with an oxide coating and caused to adhere firmly to plastic materials such as polyethylene, polytrifiuoromonochloro ethylene, or polytetrafluoro ethylene. The only method heretofore known suitable for oxidizing the surface of copper sheets continuously has been a chemical oxidation which is carried out by treating the copper sheet, after thorough cleaning, with a hot, strongly alkaline solution of sodium of potassium chlorite as described in U.S. Patent No. 2,364,993.

Processes for oxidizing copper electrolytically are also well known but their use has been limited to either a batch process, or to the treatment of copper wire continuously, as described by Hurd, U.S. Patent No. 2,828,250, which issued March 25, 1958. The sheet form of copper may not be treated continuously in the same manner as wire by the Hurd process.

According to Hurd the electrolytic oxidation of copper wire may be carried out continuously by advancing the wire through the electrolyte and applying an anodic potential+ to it.

This potential must be below a certain potential initially and as it rises it is possible in the anodization process to build up continuous coatings of black copper oxide. However, a point is ultimately reached at which a passivation of the copper entering the electrolyte occurs, and thereafter the copper Wire entering the electrolyte remains unblackened. This is overcome by periodically shorting the wire to zero potential, and then again applying an anodic+ potential to the wire to resume the process.

When copper in sheet form is treated in the same manner, a similar passivation is developed. At the start of the operation, that portion of the sheet submerged in the electrolyte begins to blacken in the desired manner, and at first the following portions also blackened as they entered the electrolyte. As the cell voltage increases, however, a point is reached where passivation occurs; then the following portions of copper remain relatively clear as they enter the electrolyte although oxidation of the portions already blackened continues. Removing the voltage and shorting out the copper sheet in the manner described by Hurd for Wire does not overcome this passivation.

According to the present invention, electrolytic anodic oxidation of copper sheet is carried out continuously by alternately applying an anodic potential to cause blackening and then rendering the copper cathodic to overcome passivation. In typical operations an anodic potential increasing from zero to about 1.5-3 volts (with reference to an iron cathode) is applied for a relatively long duration, e.g., from 30 seconds to minutes, following which a cathodic polarity is applied for a short duration, e.g., from 5 to 10 seconds, or until the passive condition of the copper sheet is counteracted. The anodic potential is then reapplied. If either the anodic or cathodic potential is applied for too long a period the copper oxide has poor adhesion to the base sheet, making the copper unsuitable for bonding to the plastic material. Preferably, the cop per is advanced through the electrolyte at a rate such that each portion remains submerged through two or more complete voltage reversal cycles. If the timing of the cycling and the speed of copper travel through the electrolyte is not proper, an oxide interface will form which has a poor adhesion to the base sheet.

The electrolyte is typically a hot alkaline solution, e.g., 1025 percent by weight of sodium hydroxide in water at a temperature of 95 C. The oxide coating appears not to differ from that obtained on wire by the process described by Hurd, nor from that obtained chemically by the treatment of the copper with hot aqueous alkaline chlorite solutions, but it is not however implied that the oxide coating attained by this invention is in all respects identical with that obtained by prior art processes.

The invention is described in detail with reference to a preferred embodiment below.

In the drawings:

FIGURE 1 is a diagrammatic View of apparatus for anodic oxidation in the practice of the process of this invention;

FIGURE 2 is a graph showing the potential of the sheet during its processing, as a function of time; and

FIGURE 3 is a developed view of a strip of copper showing the development of an oxidized portion followed by a passive portion.

in a typical electrolytic oxidation system, such as shown in FIGURE 1, a sheet 16 of copper is wound from a roll 11 and passed through a series of preliminary treatments including an alkaline cleaner bath L, followed by a rinse water tank fi l, and ammonium persulfate etching solution 16, followed by a second water rinse tank 18. The surface-cleaned strip 10 then enters the electrolytic cell 2% in which electrolytic oxidation takes place, and it is then again rinsed in a water bath 22, dried in a dryer 2%, and wound up as a roll of product 26.

In the alkaline cleaner the sheet is immersed in a hot alkaline solution containing approximately four ounces per gallon of some suitable commercial cleaner, such as Oakite No. 24, for example, and after rinsing in tank 14- a fresh copper surface is formed in an etching solution, typically containing one pound or" ammonium persulphate per gallon of water and about cc. commercial sulfuric acid (66 B.) and having a temperature of about 25 C. After the second water rinse in tank 18, the sheet ll) passes over a metallic roller 23, by which electrical contact is made to it. The sheet then travels through the tank 26 where it is submerged in the electrolyte, typically a solution of sodium hydroxide in a concentration of 10 percent by weight and having a temperature of C. With a submerged length of 8 linear feet the sheet may be run at a linear rate of 2 feet per minute to provide a 4-minute dwell of each portion of the sheet in the electrolyte.

The tank Ztl is typically constructed of steel and constitutes the other electrode of the electrolytic system. Electric power is conveniently obtained from a -volt, 60-cycle source which is converted by a variable trans former and rectifier unit R to a variable direct potential which may be controlled at between 0 and +2.0 volts. The output of the rectifier unit R is fed through a reversal switch 3% controlled by a timer T and then connected with the electrodes of the cell, with the normal positive lead connecting with the metal roller 28 and the normal negative lead connecting with the steel tank 2t During each cycle the voltage of the sheet is increased from about voltage to about +1 volt over the first 30 seconds, then stepped up to about +2.0 volts over the next 30 seconds and held at about +2 volts for an additional 30 seconds, or until a him of the desired thickness has been formed. -An increase in voltage results from the change in the conductivity characteristics of the blacl ening sheet. As a copper oxide filrn builds up, the resistance of the anode increases and as a result there is also an increase in the applied voltage. This peak voltage which denotes that the copper sheet has been oxide coated may be used with an electronic sensing device to control he cycling rate or sheet travel speed if desired. At the end of the period of positive voltage, approxirnately 90 seconds is usually adequate, the polarity of the sheet is reversed to bring the sheet to a negative voltage of about 0.5 to 1.0 volt for about 5 to seconds. The sheet is then restored to its anodic potential and the cycle is repeated.

FIGURE 3 illustrates the nature of the blackening process during the period when the clean entering portion is passive. FIGURE 3 may be considered as showing the surface of the copper strip near the end of the portion of the cycle when the sheet is anodic, or during the period when the copper is cathodic. After the sheet has been made cathodic, the portion that was passive during the proceeding cycle begins to blaclren as the anodic potential is applied in the following cycle. By this process the entire sheet may be blackened quite uniformly. Copper foil treated in this manner may be combined with plastic sheets to form printed circuits in the manner described by Dahlgren in co-pending application, Serial No. 21,272, filed April 11,1960, now Patent No. 2,997,521, and assigned to the assignee of this application.

From the foregoing description, it will be appreciated that this invention provides for the first time a continua ous electrolytic method of anodizing copper sheets, useful particularly on sheets of Width in excess of 2 inches and including Widths of 3-5 to 48 inches or more. Although the invention is described with specific reference to a preferred embodiment, it will be apparent to those skilled in the art and familiar With this disclosure that numerous modifications may be made to the described process Without departing from the scope of this invention, which is not limited to the preferred embodiment described but Which to be interpreted by the scope of the claim.

What is claimed is:

The method of continuously blackening a copper sheet of Width greater than tWo inches comprising advancing said sheet continuously through an alkaline aqueous electrolyte suitable for anodic oxidation and passing a current beta eon said sheet and said electrolyte with said sheet anodic with respect to said electrolyte, increasing the voltage of said sheet from zero to about 1.5-3 volts over a period of from about 30 seconds to 10 minutes to cause anodic oxidation with resultant passivation of the copper sheet entering said electrolyte, thereafter passing a current between said sheet and said electrolyte with said sheet cathodic with res ect to said electrolyte at a voltage of between (l.5 and 1.0 volts from 5 to 10 seconds to destroy said passivation, and again passing current between said sheet and said electrolyte with said sheet anodic to con' i no the oxidation thereof and to form a continuous oxide coating thereon as said sheet passes through said electrolyte.

Odier Feb. 13, 1951 Hurd Mar. 25, 1958 

